Requirements - fix progen def name - use dashes

Fixes #2275. Reference:
http://pybuilder.github.io/documentation/external_plugins.html#.V5m-hfmANBc

Author: 0xc0170

requirements.txt

@@ -4,7 +4,7 @@ PrettyTable>=0.7.2
 Jinja2>=2.7.3
 IntelHex>=1.3
 project-generator>=0.9.7,<0.10.0
-project_generator_definitions>=0.2.26,<0.3.0
+project-generator-definitions>=0.2.26,<0.3.0
 junit-xml
 pyYAML
 requests